It has long been known that the heart muscle provides its pumping function in response to electrical events which occur within the atrium and ventricle of the heart. Conductive tissue connects the atrium and the ventricle and provides a path for electrical signals between the two areas. In a normal heart, a natural atrial event spontaneously occurs in the atrium and a corresponding ventricular event occurs later in the ventricle. Synchronized electrical events occurring naturally in the atrium and ventricle cause the heart muscle to rhythmically expand and contract and thereby pump blood throughout the body.
In a diseased heart, atrial and ventricular events may not naturally occur in the required synchronized manner and the pumping action of the heart is therefore irregular and ineffective to provide the required circulation of blood. The required synchronized activity of such diseased hearts can be maintained by any implanted cardiac pacemaker which applies synchronized stimulating pulses to either the atrium or ventricle or both.
A diseased heart may also beat unusually quickly, a condition known as tachycardia, or may lapse into a rapid, disorganized quivering known as fibrillation. The former condition is undesirable; the latter condition may be fatal. To correct these conditions, implantable cardioverters and defibrillators have been proposed. Like the related cardiac pacemaker, these devices monitor the electrical condition of the heart and provide a corrective electrical therapy to correct the improper heart function. The three functions of pacing, cardioverting and defibrillating, or any of them, may be incorporated into a single device, generically, an implantable cardiac stimulator.
Cardiac stimulators am battery powered and, consequently, have a finite life before battery depletion may be expected. In addition to the battery, other components of the cardiac stimulation system may fail, such as leads, electrodes, or other system components. As an example of another type of change, the sensitivity of a patient's heart to electrical stimulation may change over time, altering the so-called threshold level for electrical stimulation. Such change of condition requires adaptation of the therapy delivered by the implantable cardiac stimulator, either automatically or by intervention by the attending physician. In any of these situations, or others, it may be deemed desirable to alert the patient to a changed condition so that action may be taken. For example, a pacemaker may detect the approaching end of life of its battery, in a known manner. It is desirable to alert the patient to this condition. Moreover, in the case of implantable defibrillators, delivery of therapy can be traumatic. It is sometimes deemed important to alert the patient to the prospect of eminent delivery of therapy.
Cardiac stimulators which alert or warn the patient of such conditions are known in the art. For example, such a device is described by Dutcher, et al. in U.S. Pat. No. 4,140,131. In the device described by Dutcher, et al., a device-controlled switch is activated to enable a specialized electrode adjacent the pacemaker to stimulate the patient's muscles to twitch. The nature of the electrode is not described in detail, but Ferek-Petric, in U.S. Pat. No. 5,076,272, described the electrode of Dutcher, et al., as an auxiliary electrode surrounded by the indifferent electrode and fixed on the pacemaker can. In contrast, Ferek-Petric describes a cardiac stimulator with patient warning with an electrode affixed to the header of the stimulator. Another electrode is described in U.S. patent application Ser. No. 08/426,949, filed Apr. 21, 1995, by some of us (Paul and Prutchi), also assigned to Intermedics, Inc.